A C30-6-023 has a linear excursion of +/- 1.1mm, a Fs of 570hz, is ferrofluid filled, is rated 120w power handling; Accuton recommand to cut it over 1800hz, saying nothing about the slope.

Regarding its x-max of 1.1mm, its Sd of 8.55cm² and impedance of 7.5 ohms at 1.5khz, it should theorically be able to give 109dbs / 1meter into free space (add 6dbs into half-space) at 1.5khz just before reaching its x-max. So 97dbs at 750hz (-12dbs), 85dbs at 375hz (-12dbs).

So this driver can theorically be cuted second order acoustic at 1.5khz if you project to use it half-space at max 115dbs / 1m; or still 2nd order at 2khz at max 120dbs; or still 2nd order at 2.5khz at max 124dbs. All these data are x-max related, not power compression. Nevertheless you see that the statement "It is very dangerous to crossover the ceramic tweeter below 3KHz by 2nd-order filter." isn't right in this case.

In its Humble Statement Tony Gee crosses a C30N-6-24 over at 2khz 12db/oct without any problem:

Humble Homemade Hifi